1. Field of the Invention
The present invention relates to a tracking control device for a magnetic recording and reproducing apparatus.
2. Description of the Prior Art
When a magnetic recording and reproducing apparatus (hereinafter referred to as a VTR) reproduces an information signal recorded on a recording track, tracking control is required for the reproducing head to scan the track along same for reproduction.
In one practically used method of tracking control, a special-purpose control track is provided extending in the direction of extension of a tape, a control signal is recorded at a period of a frame or of an integer times the frame period when a video signal is recorded and tracking control is performed using the control signal in reproduction. However, this method has the disadvantage that it requires the special-purpose control track and that it cannot obtain a tracking error signal over the entire region of the recording track.
Another practically used method is to get a tracking error signal, for example, by detecting the difference in reproduction time between horizontal synchronizing signals output from two heads having different azimuth angles, and generally referred to as an azimuth time difference detecting system. According to this method, a tracking error signal is obtained over the entire recording track, so that if the head is mounted on an electrical-to-mechanical converter element including a piezoelectric device and the mechanical position of the head is changed using the tracking error signal, a control system which is capable of tracing a curve in the track can be constructed.
FIG. 26 illustrates a head actuator using a piezoelectric device. The actuator includes a pair of piezoelectric devices 102 and three electrodes 101 which cooperate to hold the pair of piezoelectric devices 102 such that each device is held between adjacent electrodes 101. A magnetic head 103 is attached to an end of the head actuator. When a voltage is applied across electrodes 101, the piezoelectric devices 102 are deformed and the magnetic head 103 is displaced in the directions of double-headed arrows 104.
The azimuth time difference detecting system is disclosed in Japanese Patent Publication Sho 55-150129 and uses the relationship between a track deviation in azimuth recording and the difference in reproduction time between reproduced signals. The basic concept of this method will now be described. FIG. 27 shows a magnetized track recorded by two heads different in azimuth angle. In FIG. 27, reference numeral 201 denotes a magnetic tape moved in the direction of arrow 202; 203 and 204, A and B heads different in azimuth angle and scan magnetic tape 201 simultaneously in the direction of arrow 205. Such heads which scan simultaneously and differ in azimuth angle are hereinafter referred to as a head pair. Reference numerals 206 and 207 each denote a head gap in the corresponding head; A1, A2, A3, . . . denote magnetized tracks recorded by a head having the same azimuth angle as A head 203; B1, B2, B3, . . . denote magnetized tracks recorded by a head having the same azimuth angle as B head 204. A pair of tracks A1 and B1 and a pair of tracks A2 and B2 may be recorded by the same head pair or different head pairs. That relationship is determined optionally by the rotational speed of a rotary cylinder including a rotary head and the number of head pairs provided in the design of the device. A magnetized pattern of a signal, for example, of a horizontal synchronizing signal recorded on each magnetized track is recorded at an angle or at an azimuth angle to the direction of track extension as shown by 208 and 209.
FIGS. 28(a)-(c) illustrate the positional relationship between a recorded magnetized track and a reproducing scanning head. In these Figures, reference numerals 301, 302 and 303 shown by a broken-line block denote a head pair including A and B heads. Each head pair scans in the direction of arrow 304. Reference characters A1, B1 denote magnetized tracks recorded by A and B heads, respectively. Reference numeral 305-310 denote signals which indicate the positions where the horizontal synchronizing signal is recorded. The position of the head pair deviates leftward relative to the recorded magnetized track in FIG. 28(a), is on the track in FIG. 28(b) and deviates rightward in FIG. 28(c). When the head pair having such relative positional relationship scans a recording track, even the signals recorded at the same time differ in reproduction timing. For example, in FIG. 28(a), the horizontal synchronizing signal 305 is reproduced by the A head later than when the horizontal synchronizing signal 306 is reproduced by the B head. In FIG. 28(b), the horizontal synchronizing signals are reproduced simultaneously by the A and B heads while in FIG. 28(c) the horizontal synchronizing signal 309 is reproduced by the A head earlier than when the horizontal synchronizing signal 310 is reproduced by the B head. Therefore, the tracking error can be detected by checking the difference between the moments when the horizontal synchronizing signals are reproduced by the A and B heads.
FIG. 29 is a block diagram of a tracking error detector. Horizontal synchronizing signals (H-Sync(A)) 403 and (H-Sync(B)) 404 output from the corresponding heads are input through terminals 401 and 402, respectively, to a reproduction time difference detector 405 which detects the difference in arrival time between the signals 403 and 404 and outputs a time difference signal 406 indicative of the time difference. A tracking error calculating circuit 407 outputs from a terminal 410 a tracking error signal 411 which drives a piezoelectric device so as to minimize the difference between the time difference signal 406 output from the reproduction time difference detector 405 and a tracking reference signal 409 from a terminal 408.
However, this conventional structure includes no means to detect the absolute positions of the magnetic heads mounted on the piezoelectric devices. Therefore, it cannot detect the scanning positions of the magnetic heads. In addition, the piezoelectric devices are not uniform in characteristic and the magnetic heads may be attached at different heights on the piezoelectric devices. Therefore, the voltage applied across the piezoelectric devices and the absolute positions of the magnetic heads are not in one-to-one correspondence, so that the scanning positions of the magnetic heads cannot be obtained from the voltage applied across the corresponding piezoelectric devices. It will be described in more detail with reference to FIG. 30 which schematically illustrates a tracking pattern drawn by two head pairs. A VTR is considered in which head pairs 505 and 506 mounted on the corresponding piezoelectric devices and spaced through 180 degrees on a rotary drum scan a recording track 501. During reproduction, head pair 505 scans a pair of recording tracks A1 and B1 simultaneously, head pair 506 then scans a pair of recording tracks A2 and B2 to thereby obtain a normal reproduced image whereas as shown in FIG. 30, the scanning position of head pair 506 cannot be detected even if head pair 506 scans recording tracks A3 and B3 which are not regular scanning tracks A2 and B2. Therefore, the output timing of the reproduced video signal differs from a normal one and the original image cannot be reproduced.
When head pairs 505 and 506 mounted on the corresponding piezoelectric devices scan recording track 501 for reproducing purposes, the relationship between a scanned track and a reference (REF) signal which changes high to low or vice versa in each field may be as shown in FIG. 31(b) or (c) since the scanning positions of head pairs 505 and 506 cannot be detected although the relationship shown in FIG. 31(a) is normal. Thus, the output timing of the reproduced video signal differs from a normal one and hence the original image cannot be reproduced.